The present invention is directed to an hydraulically operated linear actuator and an electrical control system, and more particularly in its preferred form to a hydraulically operated needle bar shifter for a textile tufting machine and an electrical control system for generating control signals for commanding stepwise positioning of the needle bar, including a split phase amplifier that provides independent gain control of opposite polarity signals that produce opposite directional movement of the needle bar.
Prior art hydraulically operated linear actuators take various forms, usually having an operating rod that is reciprocated by hydraulic forces directly controlled by some type of control valve. Typically, the control valve is a servo valve that controls the direct application of the hydraulic fluid to the rod, providing positive control with relatively smooth acceleration and deceleration. However, with the servo valve normally controlling the operating hydraulic fluid directly, the size and capacity of the servo valve must be capable of controlling the pressures and flows necessary to manipulate the rod under load and with heavy duty actuators a heavy duty servo valve would be necessary with attendant relatively slower response and lesser sensitivity than is possible with low energy servo valves. In contrast, the present invention provides an indirect control of the hydraulic manipulation in which a light duty servo valve can be used with sensitive control and fast response at low energy and pressure to manipulate a rod under high energy and pressure for manipulation of a relatively heavy load.
Prior art electrical control systems of the type used to control positioning of loads as by control of a linear actuator, commonly provide control signals to manipulate a load to a preselected position that must be or inherently is within the manipulating limits of the actuator or other device being operated and that is capable of providing a repeating pattern of manipulation. By the present invention the electrical control system is capable of limiting the increment of movement within a predetermined magnitude range short of a preselected position that may be outside the predetermined range, with the subsequential incremental movements progressing toward the preselected position until the position is reached, and it further provides for a random selection of a position to which the load is moved when no position has been preselected.
Further, in many reciprocating devices, such as linear actuators, the resistance to movement in one direction is not necessarily the same as in the opposite direction so that applying the same actuating force in both directions will result in non-uniform response time and sensitivity, which may be disadvantageous as, for example, in using the actuator to shift the needle bar of a textile tufting machine laterally in the limited time available during the portion of each tufting cycle when the needles are withdrawn from the fabric being tufted. By the present invention, a split phase amplifier is used in the electrical control system to compare the magnitude of the resultant control signal with the magnitude of a signal indicating that preexisting position of the device, and imposing a polarity on the difference depending on which signal is greater, with the polarity determining the direction of movement responsive to the control signal; and separate gain controls are provided for each polarity to allow independent adjustment of the excitation level, thereby providing the capability of offsetting differences in resistance to movement in opposite directions so that uniform equally responsive movement is obtained in both directions.
In the preferred embodiment, the present invention is in the form of an electrohydraulic linear actuator for transversely shifting the needle bar of a multiple-needle textile tufting machine. There are two primary purposes for incorporating a mechanism to obtain needle bar shifting. One is the obvious purpose of producing patterns in the tufted product and the other is to minimize streaking of color shades that can result from different dye affinity of yarns when the product is dyed after tufting. To reduce inventory requirements, many tufted products which are intended to be of a single color are tufted with undyed yarn and later dyed in a selected single color as orders are received. When this is done, some of the multitude of yarn ends that are creeled for a run of a tufted product may not accept the dye equally, which is not apparent until after the product is finished. If the product is "straight stitched", streaks may be visible which make the product less acceptable. It is known to minimize this problem by breaking up the straight line stitches by transversely shifting either the needle bar or the backing fabric to produce a zig-zag pattern.
In the past, needle bar shifting has been accomplished by mechanical devices or by an electrohydraulic apparatus, such as shown for example in Schmidt et al U.S. Pat. No. 4,173,192. Such needle bar shifting in the prior art has been limited by the distance, in increments of needle gauge, over which the needle bar may be shifted in each step and by the time required to make the shift, which limit the speed at which the tufting machine may be operated to allow the necessary shifting. Further, the needle bar may, of course, be shifted only during that portion of the stitch period during which the needles are out of the backing of the product that is being tufted. This portion of the stitch period is determined principally by the total reciprocated travel distance of the needles, and by the depth of penetration of the needles into the backing material, and may be a minor fraction of the total stitch period. To function properly, the mass of the needle bar must be accelerated from zero transverse velocity to some maximum and then decelerated to zero velocity at a closely defined position all within this fraction of the stitch period. The average velocity required to shift the mass of the needle bar increases linearly with both the stitch rate and the length of shift, with the energy of motion of the mass increasing with the square of the velocity and hence with both the square of the stitch rate and the square of the shift length. Therefore, prior attempts to increase either the stitch rate or shift length have required increasingly larger cylinder areas and fluid flow rates, and when a servo valve is used, the size and the fluid pressures required may be beyond accepted commercial practice. In particular, large servo valves inherently have a longer response time than smaller valves and practical limits in the stitch rate and shift length restrict the speed at which shifting can be accomplished, which correspondingly restricts the rate at which the tufting machine can operate.
The aforementioned Schmidt U.S. Pat. No. 4,173,192 typifies the limitations of the prior art in that the electrohydraulic apparatus of that patent utilizes the servo valve directly to manipulate the operating rod to which the needle bar shifting mechanism is secured, thus requiring a servo valve of sufficient pressure and volume capability to directly manipulate the needle bar with the aforementioned resultant speed and sensitivity limitations. In the Schmidt patent a conventional closed loop control system is utilized having a feedback loop that is closed by a linear variable differential transformer, the core of which is attached directly to the operating rod to supply a feed back signal. This position signal is summed with a position command signal taken from a pattern program stored in a programmable read only memory (PROM) as a binary number that is decoded by a multiplying digital-to-analog convertor whose reference voltage is derived from the same signal that drives the aforementioned transformer. The difference between the feedback signal and the command signal is then demodulated and conditioned to a value necessary to drive the servo valve, as is familiar to those skilled in the electrohydraulic servo valve art.
In the electrohydraulic needle bar shifter form of the present invention the servo valve is independent of the operating rod manipulating hydraulics so that a high speed, sensitive response, low pressure and volume servo valve can be utilized with resulting high speed and sensitive operation of the shifter. As a result of the fast operating characteristic of the present invention, it is possible to provide a wide range of movement capability not possible with the prior art, and in this regard the system incorporates means for limiting the magnitude of movements to a selected range and also provides for random position selecting, neither of which capabilities are included in the Schmidt patent or other known prior art. In addition, the prior art does not disclose independent gain control adjustment for excitation of the servo valve in opposite directions for balancing of differences in resistance to movement in the opposite direction so that uniformity of movement is obtained.